1. Field of the Invention
This invention relates to orbital angular momentum (OAM) diverse signal processing using classical beams for applications in which OAM signal character is controlled such as optical tagging and applications in which OAM signal character is not controlled such as clutter mitigation and interference cancellation.
2. Description of the Related Art
Polarization diverse signal processing using electro-optical (EO) and radio frequency (RF) beams is used in applications such as “tagging” in which the polarization signal character is controlled and object detection, identification and tracking in which the polarization is affected by the object and its surrounding environment. The polarization state of the reflected probe signal is post-processed to identify a ‘tagged’ object or to detect, identify, track etc. an ‘untagged’ object.
FIG. 1 illustrates the functional components of a generic passive electromagnetic tagging system. There are three primary components: (i) the transmitter 10 which sends out the electromagnetic probe signal 12; (ii) the passive tag 14 on a target 15 that captures the probe signal, possibly alters the signal in a well defined manner, and transmits the (altered) return signal 16; and (iii) a reader 18 that receives the return signal, post-processes it to boost the signal-to-noise ratio and decodes the carried information. In many systems, the transmitter 10 and reader 18 are co-located on the same platform so that the reader lies in the direct return path of return signal 16. However, the return signal 16 can be received along other return paths making it easier for ‘unfriendly’ receiver to intercept the return signal. Passive tagging systems operate most commonly at radio (approximately 30 MHz to 3 THz) and electro-optical (visible band (VB) and infra-red (IR) spanning approximately 0.3 THz to 300 THz) frequencies.
FIG. 2 shows one realization of a passive radio frequency (RF) tag system. The electromagnetic emitter is an RF antenna 20 capable of producing a coded transmitted waveform 22. The passive RF tag 24 on target 25 usually has electronic circuitry that provides a varying degree of security and covertness. Receive and hold circuitry preserves the coded transmitted waveform in the coded return signal 28. Enhanced security can be realized with signal imprint circuitry that imprints a known time-varying signal onto the received RF signal or by changing the received RF waveform code in a specified manner to produce the coded return signal 28. The initial coded waveform and additional signals may be coded into the polarization or using other means. Covertness is typically achieved by spreading the RF energy across multiple RF wavebands (spread spectrum). The reader is an RF antenna 32 that uses matched filtering/pulse compression to recover the spread-spectrum RF return. Additional signal processing is possibly included to further decode the signal.
FIG. 3 shows one realization of a passive electro-optical (EO) tag system. There are a wide variety of extant passive EO tag systems. FIG. 3 illustrates an implementation based on the polarization of the EO signal. Here, the emitter is a laser 34 that transmits a signal 36 having well-defined polarization. At the EO tag 38 on target 40, a quarter-wave plate 42 alters the polarization of the received EO beam in a well-defined manner, and a retro-reflector 44 provides a simple mechanism for producing the return EO signal 46. The reader 48 consists of some optical elements 50 that preprocess the return EO signal, a Focal Plane Array (FPA) 52 for collecting the return EO signal, and possibly additional signal processing 54 to further decode the signal.
FIG. 4 shows one realization of a polarization-diverse antenna system. In this case, the depicted antenna 56 has two transmit/receive channels, each capable of processing both degrees of polarization. The transmitter 58 transmits a vertically polarized beam 60 from both channels. A target scene 62 is probed by the antenna. The scene contains a man made object 64 whose returns 66 are strongly polarized in the vertical direction as well as more natural clutter 68 whose diffuse returns 70 are more homogeneously polarized in both the horizontal and vertical directions. The antenna receiver 72 measures both polarization components of the return signal. The polarization components are processed to detect, identify, track etc. man made object 64.